The sensitivity of the human skin to the ultraviolet rays (UVR) of the sun is determined by the amount of the pigment melanin contained within it. Many individuals with fair or light/white complexions (Skin Types I, II, III) burn because they do not produce sufficient melanin to protect the skin against sunburn. Moderately brown to darkskinned persons (Skin Types IV, V, VI) are not entirely protected from the deleterious effects of solar radiation. Skin Type I--always burns easily (freckles) and never tans; Skin Type II--always burns easily and tans minimally; Skin Type III--burns moderately and tans gradually; Skin Type IV--burns minimally and tans well; Skin Types V and VI--tan profusely but rarely burn.
In addition to sunburn, long-term exposure to the sun, particularly for individuals who do not produce sufficient melanin such as Skin Types I, II, III can lead to premature aging of the skin and cutaneous cancer, usually basal cell, squamous cell carcinomas and malignant melanomas. Darkskinned persons do develop skin cancer but in small percentages, for example, malignant melanomas may occur in areas of the body where melanin is least, such as the palmar surfaces of the hands and the plantar surface of the feet. Consequently, allergic reactions, coarseness, dryness, mottling, flaccidity and blemishes are also seen. To obviate these detrimental effects, experts in the field include various combinations and percentages of chemical, physical and natural sunscreens, with the sun protective factor (SPF) ranging from 2 to 30, that is, minimal sun protection to ultra sun protection. Further, melanin precursors--tyrosine, tyrosinase and 3,4 Dihydroxy Phenylalanine (DOPA) are included in suntan preparations to stimulate the production of melanin: ##STR1## Yet, each year these harmful or life-threatening toxcities are increasingly becoming more widespread because the problem still exists for those persons who do not genetically possess sufficient melanocytes (pigment cells):
The pigment cell colors the skin by injecting melanosomes into keratinocytes. The keratinocyte carries its burden of pigment to the stratum corneum where it is shed as melanin dust. Melanin provides effective protection against actinic damage of the sun. Notably, there exists an increased correlation between skin sensitivity to UVR and melanin content. The degree of sunburn reaction, prevalence of abnormal photosensitivity and the degenerative (aging) and neoplastic changes are reduced with increasing melanin pigmentation. This increased relationship is correlated to the distribution of melansomes and quantity of melanin in the epidermis.
The SPF estimates of melanin have been cited as 1.0-4.3 to 5 for Skin Types I through Skin Types V and VI, respectively.
The photoprotective role of melanin is related to its physical and biochemical properties: melanin (a) scatters and degrades radiation to heat, (b) absorbs the radiation and promotes immediate oxidation reaction, and (c) quenches free radicals generated by UVR. Further, melanin in the human epidermis functions as a stable free radical. Because of its polyquinoid nature melanin acts as an electron exchange polymer and therefore is capable of undergoing an immediate photooxidation or darkening reaction. Melanin quenches the formulation of other types of damaging free radicals in the human epidermis upon exposure to UVR. This property of melanin to serve as a scavenger for damaging non-melanin free radicals may significantly contribute to its photo-protective role in individuals of Skin Types IV, V and VI.
However, the exposure to UVR itself, produces a phototherapeutic advantage. Subsequent to three exposures, the Type IV, V, VI skin become less likely to sunburn. However, Type I, II, III individuals develop very few melanized melanosomes. A melanin filter never develops in the stratum corneum resulting in an absence of melanin dust in the epidermis. Therefore, the need exists for the formulation of the topical application of melanin to protect the human skin from the UV rays of the sun.